Flake metallic pigment milling



Sept. 8, 1970 D. s. JACKSON FLAKE METALLIC PIGMENT MILLING 2 Sheets-Shem2 Filed May 15, 1967 mw STORQ MNW .u uk

M QQEQ INVENTOR Donald Stanley JACKSON A TTORNEY United States PatentUS. Cl. 241-24 4 Claims ABSTRACT OF THE DISCLOSURE To produce classifiedflake metallic pigments, coarse particles of metal are comminutedcontinuously in an inert liquid and the comminuted product separatedinto at least three fractions, and at least the separated fractioncontaining the coarsest size particles and liquid recycled to thecomminuting operation and the remaining fractions either processed torecover the flake metallic particles from the liquid or recycled. Theparticle size of the separated fractions below the coarsest ispreferably below the range from 300 to 100 mesh. At least one of thefractions not recycled to the comminuting operation ma be sub-classifiedinto at least two fractions of different particle size distribution,from which flake metallic particles are recovered.

This invention relates to the production of well classified flakemetallic pigments.

Metal powders or pastes serving as pigment for paint are genericallyreferred to as bronze powders or bronze paste even though made frommetals or alloys of metals which do not have or give what isspecifically known as bronze appearance. The expressions bronze powderor bronze paste and the expression bronze paint as herein used are eachemployed in the broad generic sense as including any of the metallicpowders, pastes or paints, even though the same is used and relied uponto give a golden, yellow, silvery or other metallic appearance to anarticle or surface coated thereby. The present invention ischaracterized by being economical of operation and capable of producinga more classified pigment than other known methods of manufacture. Otheradvantages will become apparent upon description of the process.

Bronze powders are manufactured either from alloys of copper such asbrass, in cases where a golden or yellow tint is required or fromaluminum, where the paint is to have a silvery cast. Such presentpowders as above mentioned are made by either stamping the metal insmall hammer mills and subsequently polishing the product so obtained byrotating brushes, or other devices with the addition of a small quantityof suitable lubricant, or by milling in a grinding medium consisting ofan inert liquid with subsequent separation of the liquid grinding mediumto the extent desired depending on whether a paste or dry powder isrequired. Characteristic processes are disclosed in US. Pats. 1,930,684;1,954,462; 1,832,868; 2,112,497; 1,498,318; 1,932,741; 2,136,445 and2,002,891. Polishing of pastes produced by the wet comminution processmay be accomplished by methods such as that described in US. Pat.2,591,245. Or, if the liquid medium is completely separated, by methodssuch as those described for stamped powders. By any of the methodsdescribed, pastes or powders of two distinct types may be produced. Bychoice of milling and polishing lubricants either a lac or leafing typeor a non-le'afing type can be produced. The term lac or leafing refersto the property of the flake metal particles to orient themselves on thesurface of a suitable vehicle to produce a mirror-like metallic finish.The scope of this invention is understood to encompass both of thesetypes of products.

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In all of the above described methods of manufacture, classification ofthe primarily produced comminuted flake product is commonly employed.The purpose of classification in these processes is to return to thecomminution circuit coarse material produced at the same time so as toallow continuous operation of the comminution device. The pigment soproduced tends, however, to induce an objectionable grey or blue castwhen used as a pigment for many applications. This arises becauses theeffect of classification as normally employed is to control the uppersize limit with no etfect on the distribution of particle sizes belowthe sieve cut-off point. This is especially true in the sub-sieve rangeof particle sizes. The quantity and size distribution of fines can becontrolled within certain limits by increasing the rate of circulationof product and return of oversize around the milling classifyingcircuit. There are, however, practical limits to this approach as theclassifying equipment will become overloaded or classifying equipment ofextremely large capacity will be required.

Having regard to the foregoing, it is an object of this invention toprovide for the manufacture of bronze powder or paste with a morecontrolled particle size distribution than can be manufactured by theconventional processes in a convenient manner.

It has generally been found that fine aluminum pigments which arecapable of passing through a 325 or 400 mesh screen are quite grey inhue. Pigments produced according to the teaching of this invention havea controlled particle size distribution and are much more brilliant andmetallic in lustre and exhibit superior color purity. Polychromaticpaint films prepared according to conventional formulations, whenpigmented with aluminum paste produced according to this invention, whencompared to similar films pigmented with aluminum pigment produced byconventional processes, were superior in gloss, brilliance and colorpurity, and exhibited higher reflectance as measured on a ColormasterTri-Stimulus Colorimeter.

These results satisfy the general requirement that has emerged formetallic pigments of a classified character for such applications aspolychromatic finishes or the pig mentation of plastics. See SpecialEffects December 1961, Modern Plastics and Evaluation and Description ofMetallic Colors, Society of Plastics Engineers Journal, volume 21, No.12, 1965.

According to the invention coarse particles of a metal of the kind usedfor pigments are subjected to a Wet com minution operation in a grindingcircuit in which the metallic particles are maintained within an inertliquid. Liquid bearing the comminuted particles is withdrawn from thecomminuting operation and classified into at least three fractions beingtwo oversize fractions of different particles sizes and an undersizefraction. The fraction containing the coarsest particles is returned tothe grinding circuit. The fraction containing particles of anintermediate size is removed from the circuit. The fraction containingthe finest particles is preferably isolated and the particles separatedfrom the liquid as an end product. The particles from the fraction ofintermediate size as well as those from the fraction of finest size areusable as pigments.

In accordance with the invention, a further separation may be eifectedin which at least one of the fractions not returned to the grindingoperation is subjected to further classification. In this operation thefraction is separated by another type of classifying operation from thatused for the primary classification for example, differential settling,so as to obtain two or more fractions of narrower particle sizedistribution than the original fraction.

The invention will be described in more detail by reference to theaccompanying drawings illustrating preferred aspects of the inventionand in which,

FIG. 1 is a diagrammatic representation of apparatus suitable forcarrying out the invention;

FIG. 2 is a diagrammatic representation of a type of classifier used insub-classification of the finest particle size fraction from theapparatus of FIG. 1;

FIG. 3 is a diagrammatic representation of characteristics of a priorart product produced when all of the oversize is returned to the milland the undersize is the sole end product;

FIG. 4 is a diagrammatic representation of characteristics of the fineproduct produced when an intermediate oversize product is removed andnot returned to the mill.

FIG. 5 represents characteristics of the product resulting from thefurther classification of the product of FIG. 4;

FIG. 6 represents the characteristics of a product obtained by furtherclassification of the product of FIG. 4 operated in a somewhat differentmanner from that of the procedure resulting in the product of FIG. 3.

More specific reference will now be made to the drawmgs.

A multideck classifying screen of a vibrating type is shown at 15. Thescreen is provided with respective screens 15a, 15b and 15c. Cut foilscrap or other suitable feed and lubricant is fed to a ball mill 16through a feed line 17 from a suitable source of supply (not shown)together with a solvent make-up proceeding through line 19 from a filterpress 21 which acted to remove the solvent makeup from an already milledand screened fraction. The ball milled material from the mill 16 passesthrough the line 23 to the multideck screen 15. Oversize material passesfrom the tops of the screens 15a and 15b to the lines 25 and 26 andthence into an oversize return line 27 back to the ball mill 22.

An intermediate size fraction passes from the screen 150 through theline 29 to the filter press 21, where liquid is removed and recycled inthe line 19 or reclaimed in the line 31. Undersize material passesthrough the line 33 also to a filter press 35 which separates the solidsfrom the liquid.

The undersize solid product separated from the liquid by the filterpress 35 and the intermediate product from the filter press 21 representend products which are each usable as pigments.

Should it be desirable to sub-classify the undersize material instead ofbeing taken to the filter press 35 it is lead to a differential settler37. This device separates the undersize fraction into two sub-fractionswhose constitution can be adjusted depending on the flow rate.

The choice of the intermediate screen 150 cannot be specified exactlybecause it depends on the mechanical construction of the screen and thesieve, and the methods of vibrating. The choice of the intermediatescreen has to be made on the basis of examining the particle sizedistribution of the intermediate fraction and of the finest particlesize fraction. A guide line in selecting the screens of effective sizesis that an attempt must be to cut the milled product into fractions ofas close the same weight as possible. Generally speaking theintermediate screen may range between 300 mesh and 100 mesh.

Typical procedures will now be exemplified for the further explanationof the invention, by reference to the following examples.

EXAMPLE I A ball mill with dimensions 3 feet long by 3 feet diameter wascharged with 2,200 lbs. of steel balls of mixed size from 4 inchdiameter to /2 inch diameter. Under equilibrium operating conditions aslurry of Varsol and aluminum flake powder containing 23% solids Wastransported at the rate of 172 lbs/hr. to a multideck vibratoryscreening device (30 inch) fitted with screens of graduated finenessfrom 80 mesh on the top deck to 325 mesh on the bottom deck. The productwas the minus 325 mesh portion and was produced at the rate of 6lbs./hr. of metal solids. All of the oversize was returned to the milltogether with 6 lbs. of raw cut foil scrap to maintain the circulatingmetal content of the system constant. Quantities of Varsol and lubricantwere added sufficient to maintain a material balance in the system. Theproduct was largely freed from the Varsol diluent by means of a filterpress. The Varsol so separated was used to make up the Varsolrequirements of the system. The properties of the product so producedwere given in Table I, as a basis of comparison of a prior art productwith a product of the present invention.

EXAMPLE II See FIG. 3 and FIG. 4.

A ball mill with dimensions 3 feet long by 3 feet diameter was chargedwith 2,200 lbs. of steel balls of mixed size from inch to /2 inchdiameter. Under equilibrium operating conditions a slurry of Varsol andaluminum containing 2.1 percent solids was transported at the rate of189 lbs./ hr. to a multideck vibratory screening device fitted withscreens of graduated fineness from mesh on the top deck to 3.25 mesh onthe bottom deck. The equipment was identical to that described inExample I. The primary product was the minus 325 mesh portion and wasproduced at the rate of approximately 5.1 lbs./hr. of metal solids. Aminus 250 mesh plus 325 mesh oversize portion was collected separatelyas a second product at the rate of approximately 0.8 lb./hr. and therebydiverted from the oversize returned to the mill. The remaining oversizewas returned to the. mill together with 6 lbs. of raw cut foil scrap tomaintain the circulating metal content of the mill. Quantities of Varsoland lubricant sufiicient to maintain a material balance in the systemwere added. The products so produced were largely separated from theVarsol diluent by means of a filter press. The Varsol so separated wasused to make up the Varsol requirements of the system. The properties ofthe minus 325 mesh product are given in Table I.

EXAMPLE III The minus 325 mesh product of Example II after passingthrough the 325 mesh screen and before separation from the Varsoldiluent was passed through a settling device similar to that revealed inCanadian Pat. 554,038 (see FIG. 5) at a rate sufficiently great toproduce two fractions, a coarse underflow fraction and a finer overflowfraction. The flow rate through the settling device was controlled tosplit the original minus 325 mesh product into an overflow fractioncontaining 10 percent of the original pigment and an underflow fractioncontaining percent of the original pigment. The properties of thesefractions are given in Table I.

EXAMPLE IV The minus 325 mesh product of Example II, after passingthrough the 325 mesh screen and before separation from the Varsoldiluent, was passed through a settling device as described in ExampleIII at a rate greater than that used in Example III which was sufiicientto produce an overflow fraction containing 50 percent of the originalpigment and an underflow fraction containing 50 percent of the originalpigment.

The data of Table 1 reveals particle size distribution data obtained bymicroscopic examination of the pigments produced. This data was producedby dispersing the pigments in a collodion film, counting 1000 particlesand classifying these particles into various size categories. Thesecounts were reduced to percentage distributions based on particle size.While one can see that there has been, in general, a redistribution ofparticle sizes between the pigment of Example I and Example II it is notvery apparent that this should lead to the noted improvement intri-stimulus reflectivity. It is perhaps more apparent if this data isrecalculated to show the relative quantity of reflective surfaceavailable in each particle size range. This can be done by multiplyingthe particle size count in each size range by a factor proportional tothe square of the average particle size. This assumes that thereflective surface available from these flake pigments is proportionalto the area of one surface only. The derived reflective surfacefunctions for each pigment can be plotted to show a cumulative increasein reflective surface up to 100 percent for the whole pigment and adifferential distribution of reflective surface derived as a function ofparticle size for the different size ranges. This latter data are shownin FIG. 6.

It is apparent from the data of FIG. 6 that real differences existbetween the pigment produced according to Example I and that producedaccording to Example II. While both pigments can be classified as minus325 mesh pigments, that of Example I derives its reflectivecharacteristics from two size classifications of particles and that ofExample II is mono disperse. The visual effect produced from pigmentproduced according to the teaching of Example II is much cleaner withless suggestion of fine particle background behind the coarserparticles.

It is possible to improve the degree of classification when operatingaccording to the teaching of Example I by increasing the rate ofwithdrawal of metal Varsol slurry from the mill or comminution device.By this means one can produce pigments intermediate in character betweenthat produced according to the teaching of Example I and that producedaccording to the teaching of Example II. However, to accomplish this oneis required to utilize excessively large classifying equipment in orderto handle the flow of metal-Varsol slurry. It is much more convenientand economical to achieve the desired classification by withdrawing anintermediate oversize product containing metal flakes somewhat coarserthan the desired product. By this means overmilling is avoided. Inaddition it has been found that the coarser fraction (minus 250 meshplus 3.25 mesh) product has considerable utility as a pigment and is adesirable product.

available screens. However, the same technique can be used if desired todifferentiate the minus 250 mesh plus 325 mesh fraction into twoderivative fractions.

It is not necessary to utilize a screening or sieving classification toachieve the purposes of this invention. It is obvious to anyone skilledin the .art of classification that any type of classifier capable ofproducing at least three well defined products (in terms of particlesize) from the primary mill output would be satisfactory. See forexample Wilfiey table and similar devices described in Handbook ofMineral Dressing Taggert, Wiley & Sons, 1945, Sections 11-62 to 11-90.Other types of classifiers may be used providing they meet therequirement of producing sulficient differentiation of particles basedon flake diameter and sufficient fractions thereof. Similarly the scopeof this invention is not limited to the use of ball mills. Anycomminution device capable of applying impact pressure and frictionalrubbing contact sufficient to break up and flatten out the metalparticles and s completely to change their form into flake-likeparticles, will suffice for the practice of this invention. Similarlythis invention is not restricted to the use of an overflow classifier ofthe type cited for secondary classification of the sub-sieve product.Any type of classifier capable of differentiation of the solvent borneflake particles in what is commonly known as the sub-sieve range ofparticle sizes into two or more fractions will suffice, e.g. liquidcyclone. Some such devices are cited in Handbook of Mineral Dressing,Taggert, Wiley & Sons, 1945, Sections 8, 10 and 11.

The practice of this invention leads to a logically related series offlake metal pigments which vary in particle size from a size capable ofbeing differentiated by .a simple screening system to products in thesub-sieve range. All of these products are classified in that they havea desirable particle size distribution and as such especially valuablefor pigmentation. The relationship of the pigments to the mode ofproducing them is shown in FIGS. 3, 4, and 6.

TABLE I.PARTICLE SIZE COUNT (EXPRESSED IN MIORONS)Colormastepitri-stlmulus rea ng Below 5 5-10 -15 -20 10-20 -30 -40 -50(percent) (percent) (percent) (percent) (percent) (percent) (percent)(percent) R G B Exam lo I rior art 60.8 19. 6 14. 7 1. 9 2. 4 0. 5 35. 935.8 36. 3 Examgle II? 54, 2 26. 4 14. 7 3. 3 0. 9 0. 4 30. 8 36. 6 37.1 Eiiclample III-10% overflow, 90% under- Undcrflow (coarse) 55.5 23.515.6 3.6 1.2 0.5 37.1 37.0 37. Overflow (fine) 84.5 12.7 2.5 0.2 32.332.0 32. Eiirlample IV-% overflow, 50% under- Underfiow (coarse) 58.220.7 15.0 4.3 1.2 0 6 37.8 37.5 38. 0 Overflow (fine) 77. 2 14. 6 6. 81.2 0. 2 34. 2 33. 9 34. 5

It is apparent from the results shown in Table I and I clam:

FIGS. 5 and 6 that a further desirable classification of product in thesub-sieve range (minus 325 mesh) is achieved according to the teachingof Example III and Example IV. Depending on the extent which the minus325 product of Example II is split into overflow and underflowfractions, the degree of classification of the superfine pigment(overflow fraction) can be controlled. By these two processes alogically related and well differentiated (in terms of particle size)series of pigments can be produced. The visual optical effects producedby these various pigments seem to depend not on the location of the peakin the curves shown in FIG. 6 but on the relative quantity of reflectivesurface at the coarse and fine end. Panels prepared using the twopigments produced by the method of Example IV are very different. Theoverflow pigment appears very much finer than the underflow pigment. Theteaching of Examples 111 and IV constitute an extremely efficient methodof producing classified pigments Whose maximum particle size is muchless than the effective control aperture of commercially 1. A method formanufacturing metallic flaked powders, comprising, continuously passingan inert liquid containing coarse particles of metal through a grindingcircuit whereby they are comminuted, continuously withdrawing from thegrinding circuit liquid containing the thus comminuted particles,classifying the withdrawn comminuted material into two oversivefractions of different particle sizes and an undersize fraction which isthe desired classified product, and continuously returning to thegrinding circuit only the coarsest of the two oversize fractions.

2. A method, as defined in claim 1, in which the finer of the oversizefractions removed from the grinding circuit has a particle size withinthe range from 300 to mesh.

3. A method, as described in claim 1, in which the metal is aluminum.

4. A method, as defined in claim 1, in which the metal is a copperalloy.

(References on following page) one 7 8 References Cited "2,272,6292/1942 Arthur 2413 ENTS 2,274,766 3/1942 Ziehl 24-1-3 22 3 PAT 241 32,526,519 10/1950 Jorgensen 24120 er 5 1 4/1934 Tainton 241 3 2, 7 ,86610/1951 Greene 241 20X 313:; 1 2111 5 LESTER M. SWINGLE, PrimaryExaminer 2/1939 ZQ D. G. KELLY, Assistant Examiner

